In ball screw and machine screw jacks, such as sold by Nook Industries of Cleveland, Ohio under the trademark ACTIONJAC, rotary limit switches are commonly employed to control and regulate jack travel. Such rotary switches are mounted directly on the jacks and are worm gear operated and regulate jack travel by sensing the number of input shaft revolutions.
In such application rotary limit switches present a number of problems. A major problem is the cost. Another problem is bulk. Such rotary switches can be mounted in a number of positions but in each project substantially from the jack creating clearance problems in some installations. They of course also require a shaft drive connection. Rotary limit switches also are subject to wear, have a relatively short service life, and tend to drift as they wear, thus not providing highly reliable repeatability.
Mechanical pole arm actuated limit switches have many of the same problems, particularly when mounted on projecting brackets. Moreover, the screws of such jacks are usually sealed in a cover tube such that physical contact with the screw is not really feasible.
In such jacks the cover and screw are usually made of steel which are subject to magnetic flux making the use of low cost magnetic switches unreliable, without modification of the jack assembly. If low cost magnetic reed switches or Hall effect sensors could be employed with easy installation and adjustment and achieve reliable repeatability, a lower cost, more reliable jack assembly with longer service life can be obtained .